In certain Ka band satellite communication systems, satellite gateway outroute transmissions intended for one or more satellite terminals can interfere with inroute transmissions from satellite terminals to a satellite gateway. The inroute transmissions are affected by thermal noise effects with a spectral density of N0 Watts/Hz. The presence of outroute interference results in an increase in the spectral floor due to the undesired effects from No to No+Io (where Io refers to spectral density, or level, of outroute induced interference). The level, Io, of the interference can vary with varying center frequencies and bandwidths of the outroute signals (i.e., the interference floor Io as a function of the frequency can be uneven). Inroutes on certain transmission frequencies experience more interference compared to inroutes on some other transmission frequencies.
In certain existing satellite communication systems, spectral density, or level, of noise plus interference No+Io is assumed to be identical across all frequencies. During initial installation and commissioning, the satellite terminal performs a procedure, subsequently referred to as a ranging procedure, to account for the No+Io level, in addition to a nominal value of end-to-end channel gain. The satellite terminal transmits the ranging signal at one frequency. Based on a received power level PRX, of the ranged frequency received at the satellite gateway, the satellite terminal determines a nominal transmit power level (the ranging power setting). The certain existing satellite communication systems assume that the spectral density, No+Io, is flat across all inroute frequencies. As a result, the nominal transmit power level estimated at the ranging frequency is used with respect to other inroute frequencies.
During the ranging process, the satellite terminal transmits a ranging signal at a maximum power level. The satellite gateway receiver measures a signal quality of the ranging signal and the satellite gateway transmits a message to the satellite terminal containing a measured Signal Quality Indicator (SQI). The SQI, as received by the terminal, typically exceeds a Signal Quality Target (SQT). The satellite terminal subsequently reduces transmit power and retransmits the ranging signal. The ranging process is repeated until the SQI received by the terminal from the satellite gateway approximately equals the SQT. When this occurs, the ranging process is declared to have been converged, and the terminal stores the final transmitted power upon the convergence of the ranging power. When the ranging process is performed during a clear sky condition, the ranging power value is determined by a clear sky path loss, satellite and ground equipment hardware gains, and the noise and interference N0+I0 spectral density.
Subsequent to ranging, when the satellite terminal initiates a transmission, the satellite terminal determines a power level with which to begin the transmission. The ranging power derived during an initial ranging process is a best estimate that the terminal has for the power level. The estimate is accurate during the clear sky condition and for an operating scenario in which the noise and interference (N0+I0) floor, or level, is flat (i.e., it does not exhibit variations over different inroute frequencies). It is with assumptions as described above that, in an existing Ka band system, the ranging power value derived by the satellite terminal is used as a nominal transmit power whenever the satellite terminal initiates a return uplink transmission subsequent to the ranging process.
Similar to the ranging process, the satellite gateway continually measures the SQI for each uplink transmission from the satellite terminal and sends a message containing the SQI to the satellite terminal. The satellite terminal measures the difference between the SQT and the SQI received from the satellite gateway. The measured difference is a Power Control Error or PCE.
The PCE is used as an input to a system tracking filter (STF). An output of the STF is used to adjust the transmit power of the satellite terminal, relative to the initial transmit power, which is equal to the ranging power level. A positive-valued PCE, which occurs when SQT exceeds SQI, indicates that the satellite terminal is under-powered (i.e., it is transmitting at a less than desired power level). The positive-valued PCE causes the STF output to increase, which, in turn, increases the satellite terminal transmit power. The increased satellite terminal transmit power reduces a shortfall of the SQI relative to the SQT. Similarly, a negative-valued PCE results in the satellite terminal reducing the transmit power. Thus, in general, the PCE and the STF act to balance the satellite terminal transmit power, such that it is just enough to ensure that SQI approximately equals SQT. One single STF is used across all the inroute frequencies. This is because of the assumption, stated earlier, that the N0+I0 floor is flat. With this assumption, the main variable in an end-to-end link is Ka band channel gain, which can decrease from its value during the clear sky condition. The STF compensates for variation of the channel gain.